1. Field of the Invention
The present invention relates generally to a gas turbine engine, and more specifically to a turbine airfoil with near wall low flow cooling.
2. Description of the Related Art
Including Information Disclosed Under 37 CFR 1.97 and 1.98
A gas turbine engine includes a turbine with multiple rows or stages of rotor blades and stator vanes each with an airfoil that reacts with a hot gas flow. Rotor blades and stator vanes have different design constraints due to the rotor blades being exposed to large centrifugal forces from rotation while the stator vanes are exposed to bending forces. However, both blades and vanes have airfoils that require cooling in order to withstand the high gas flow temperatures, especially for the first stage airfoils.
In order to increase the turbine efficiency, and therefore the engine efficiency, a higher gas flow temperature is passed into the turbine, referred to as the turbine inlet temperature. However, the highest turbine inlet temperature is limited to the material properties of the turbine, mainly the first stage airfoils, and the amount of cooling that can be produced for these airfoils. Since the pressurized air used for cooling of these airfoils is bled off from the compressor, the cooling air decreases the efficiency of the engine because work is performed to compressor the cooling air and no useful work are extracted from the compressed cooling air. Thus, low flow cooling circuits for airfoils has the advantage that the engine efficiency is increased.
FIGS. 1 and 2 show a prior art rotor blade using near wall radial flow cooling channels in the airfoil main body. This type of airfoil with the radial channels is constructed by means of a mini-core casting process. For the cooling of the airfoil wall, resupply holes and spanwise extending film cooling holes are used with trip strips in the near wall channels to enhance the heat transfer coefficient. with this prior art near wall cooling design, the spanwise and chordwise cooling flow control due to the airfoil external hot gas temperature and pressure variations is difficult to achieve. Also, the single radial flow channel is not the best method of utilizing the cooling air because it produces a low convective cooling effectiveness. However, the manufacturing approach to from this type of near wall radial flow cooling channel is constrained by the mini-core positioning and minimum wall thickness requirement for the molten metal flow during the casting process. Thin wall airfoils work best for near wall cooling which will maintain low metal temperatures. However, thin walls are very difficult to cast using the investment casting process because of the molten metal is of low viscosity and thus does not flow through small spaces and due to the ceramic core shifting as the heavy molten metal makes contact with the core.